1. Field of the Invention
The present invention relates to a direct-heated flow measuring apparatus having a film resistor which serves as a temperature detecting means as well as an electric heater. Such a direct-heated flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
2. Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, but this is disadvantageous in scale, response speed characteristics, and the like, and therefore, airflow meters having temperature-dependent resistors have been developed, in which these disadvantages of scale, response speed characteristics, and the like are avoided (see: U.S. Pat. No. 3,975,951).
There are two types of airflow meters having temperature-dependent resistors, i.e., the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor. In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor on the upstream side is used for detecting the temperature of non-heated air. The current flowing through the heater resistor is controlled to provide a constant difference in temperature between the two temperature-dependent resistors, and thus the mass flow rate of air is determined by detecting the voltage applied to the heater resistor.
In this heater-type airflow meter, if an upstream temperature-dependent resistor is not provided and the current of the heater resistor is controlled to provide a constant temperature of the downstream temperature-dependent resistor, the voltage applied to the heater resistor is detected as representing the volume flow rate of air.
On the other hand, the direct-heated type airflow meter may consist of a film resistor which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air. Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled to provide a constant difference in temperature between the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as representing the mass flow rate of air. In this direct-heated type airflow meter, too, if a temperature-dependent resistor is not provided and the current of the heater resistor is controlled to provide a constant temperature of the film resistor, the voltage applied to the film resistor is detected as representing the volume flow rate of air.
Since the film resistor of the direct-heated type airflow meter serves as a temperature-detecting means for heated air, that is, an additional temperature detecting means for heated air is not necessary, the direct-heated type airflow meter is smaller in size than the heater-type airflow meter.
In the direct-heated type airflow meter, the film resistor may consist of an insulating substrate such as a ceramic substrate or monocrystalline silicon substrate, a resistance pattern layer of platinum (Pt), gold (Au), etc. on the insulating substrate, and a heat-resistant resin or a passivation layer on the resistance pattern layer.
Usually, the response characteristics and dynamic range of the direct-heated type airflow meter are dependent upon the heat mass and adiabatic efficiency of the heating and temperature detecting portion of the resistance pattern layer, which serves not only as a heating means but also as a temperature detecting means. In order to obtain the most excellent response characteristics and largest dynamic range, the above-mentioned portion should be ideally in a completely floating state in the air stream.
For this purpose, there may be suggested a direct-heated airflow meter in which the substrate is fixed by an adiabatic member to a supporting member having good heat dissipation characteristics. In this case, the adhesion between the substrate and the adiabatic member and the adhesion between the adiabatic member and the supporting member are carried out by using adhesives having an adiabatic effect, thus further enhancing the adiabatic effect of the heater and temperature detecting portion. Such adhesives are, for example, resin adhesives, frit glass, and the like.
The thickness state, and the like of the above-mentioned adhesives, however, fluctuate according to the manufacturing conditions thereof, and accordingly, the adiabatic effect thereof is fluctuated. Thus, fluctuations occur in the response characteristics and dynamic range of the airflow meter.